This painting is by Luis Royo and was used as the cover for A Yoke of Magic by Robert Vardeman (which I’m currently reading).

A few quick descriptions from the novel of the riders and their mounts:

The three riders reined in and held their snorting mounts just beyond the ring of pale light cast by Lejena’s campfire. Their horses pawed the earth, searing the ground wherever they touched; their hooves were afire.

As she neared, the foremost of the demon riders grasped a longsword forged from crystalline flame. It blazed brilliantly as it slid form a dark scabbard, tiny tongues of fire leaping and dancing along its entire length.

In the light of those hell-fired swords, Davin saw the heavy cowls pulled forward about their heads. No matter which way they turned, their faces – if they had any – remained cloaked in shadow. But the hands gripping those flaming blades were skeletal!

Between the images and the description all I can think is light-sabre wielding Nazgûl.